1. Field of the Invention
The present invention relates to a circuit board including metal pieces soldered onto pads formed on an obverse surface of a substrate, and a method of making the circuit board.
2. Description of Related Art
Traditionally, a battery pack having built-in rechargeable batteries is used as a power source for a portable electronic device such as a mobile phone or laptop computer, for example. The battery pack is removable from the main unit of the portable electronic device. Generally, such a battery pack includes a resin housing enclosing rechargeable batteries and a circuit board which serves as a protection circuit for preventing over-discharge from and overcharge to the rechargeable batteries.
FIG. 5 shows a typical example of such a circuit board. This circuit board 100 includes a board substrate 110 formed with a variety of electronic components constituting a protection circuit, and terminal plates 40 for connection to a rechargeable battery 9.
The board substrate 110 is made of an insulator such as a glass epoxy resin, and the upper surface 101a thereof is formed with pads 103 made of a conductive film. The terminal plates 40 are metal pieces 4 directly soldered onto the pads 103. The terminal plates 40 (the metal pieces 4) are formed into strips, each including a free end 4b projecting beyond a longitudinal end of the board substrate 110.
For connecting the circuit board 100 and the rechargeable battery 9, the terminal plates 40 are bent so that the free ends 4b come generally perpendicular to the board substrate 110 before firmly bonding the free ends 4b to the electrodes (not shown) of the rechargeable battery 9 by e.g. welding.
However, users may drop, by mistake for example, such a portable electronic device having a built-in battery pack where the circuit board 100 is incorporated, in which case, large load is imposed especially at the joint surface between the metal pieces 4 and the board substrate 110. The metal pieces 4, soldered on the pads 103, are relatively firmly bonded to the pads 103. On the other hand, the connection between the pads 103 and the board substrate 110 is weak, because as shown in FIG. 6, the pads are formed by etching a copper foil provided on the upper surface 101a of the board substrate 110 wherein the copper foil generally includes a rough surface (mat surface) suitable for pressure attachment which is only pressed upon the upper surface 101a of the board substrate 110 for bonding. Therefore, when the load is imposed as described above, the pads 103 are completely detached from the board substrate 110, causing the metal pieces 4 to fall off.